RT7294A
Feed-forward Capacitor (Cff)
Enable Operation (EN)
The RT7294A is optimized for ceramic output capacitors
and for low duty cycle applications. However for high-output
voltages, with high feedback attenuation, the circuit's
response becomes over-damped and transient response
can be slowed. In high-output voltage circuits (VOUT > 3.3V)
transient response is improved by adding a small “feed-
forward” capacitor (Cff) across the upper FB divider resistor
(figure 1), to increase the circuit's Q and reduce damping
to speed up the transient response without affecting the
steady-state stability of the circuit. Choose a suitable
capacitor value that following below step.
For automatic start-up the EN pin can be connected to
VIN, through a 100kΩ resistor. Its large hysteresis band
makes EN useful for simple delay and timing circuits. EN
can be externally pulled to VIN by adding a resistor-
capacitor delay (REN and CEN in Figure 2). Calculate the
delay time using EN's internal threshold where switching
operation begins (1.4V, typical).
An external MOSFET can be added to implement digital
control of EN when no system voltage above 2V is available
(Figure 3). In this case, a 100kΩ pull-up resistor, REN, is
connected between VINand the ENpin. MOSFET Q1 will
be under logic control to pull down the ENpin. To prevent
enabling circuit when VINis smaller than the VOUT target
value or some other desired voltage level, a resistive voltage
divider can be placed between the input voltage and ground
and connected to EN to create an additional input under
voltage lockout threshold (Figure 4).
Get the BW the quickest method to do transient
response form no load to full load. Confirm the damping
frequency. The damping frequency is BW.
EN
R
BW
EN
V
IN
EN
RT7294A
C
EN
GND
V
OUT
Figure 2. External Timing Control
R1
C
ff
FB
RT7294A
R
100k
EN
V
EN
RT7294A
GND
IN
R2
GND
Q1
Enable
Figure 1. Cff Capacitor Setting
Figure 3. Digital Enable Control Circuit
Cff can be calculated base on below equation :
1
Cff
23.1412R1BW 0.8
R
EN1
V
IN
EN
Internal Soft-Start (SS)
R
EN2
RT7294A
GND
The RT7294A soft-start uses an internal soft-start time
800μs.
Following below equation to get the minimum capacitance
range in order to avoid UV occur.
Figure 4. ResistorDivider for Lockout Threshold Setting
COUT VOUT 0.61.2
T
(ILIM Load Current)0.8
T 800μs
Copyright 2016 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS7294A-05 October 2016
www.richtek.com
11
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